FIG. 1 shows a conventional seat frame 10 included in a seat of a vehicle such as a car. As shown in FIG. 1, the seat frame 10 comprises a seat cushion frame 20 to be included in a seat cushion (not shown) and a seat back frame 40 to be included in a seat back (not shown).
Generally, the seat cushion frame 20 has side brackets 21 and 22, which are placed opposite each other with an interval in a lateral direction. The side brackets 21 and 22 are connected by such means as connecting members 25 and 26. The side brackets 21 and 22 are mounted to well-known slide mechanisms S1 and S2, respectively. The lower arms 23 and 24 are connected to rear portions of the side brackets 21 and 22 via connecting metal fittings 27 and 28, respectively. The upper arms 32 and 33 are pivotably connected to the lower arms 23 and 24 via well-known reclining mechanisms 30 and 31, respectively.
The seat back frame 40 has a pair of side plates 41, 42 and an inverted U-shaped seat back main frame 43. The pair of side plates 41 and 42 is to be mounted to the seat cushion frame 20 via upper arms 32 and 33, respectively. The inverted U-shaped seat back main frame 43 is connected to the side plates at their tops. The seat back main frame 43 has mounting parts 44 and 45 for inserting and mounting the headrest stays (not shown).
FIG. 2 shows the conventional seat frame 10 having the seat cushion frame 20 and the seat back frame 40, shown in an assembled state.
FIG. 3 shows a side view of part of the seat frame 10. When great impact force is applied from behind to a vehicle with seat frame 10 included in the seat, such as by being rear-ended by another vehicle (hereinafter referred to as a rear-end collision), backward force F is applied to the seat back frame 40, causing a bending moment M around the lower fixed end of the seat back frame 40, as shown. If the bending moment is large, the reclining mechanism 30, which connects the upper arm 32 and the lower arm 23, and/or the connecting metal fitting 27, such as caulking, which connects the lower arm 23 and the side bracket 21, could be damaged.
Recently, there is increased demand for eco-friendly vehicles, such as electric vehicles (EV) and plug-in hybrid vehicles (PHEV), and for greater fuel efficiency of conventional, gasoline-powered vehicles. Therefore, more lightweight vehicular components are required. Connecting members such as reclining mechanisms have been lightened and downsized. However, lightened or downsized connecting members such as reclining mechanisms could be damaged when subjected to a large bending moment at the time of a rear-end collision.
If the reclining mechanism 30 or the connecting metal fitting 27 is damaged or fractured, the seat back frame 40 could collapse backward. This could be prevented by enhancing the rigidity of the reclining mechanism 30 or the side bracket 21. However, enhancing rigidity may result in a failure to reduce weight. Therefore, a vehicular seat frame structure that is both reduced in weight and capable of improving safety is required.
For ensuring safety, a seat frame capable of absorbing collision energy has previously been proposed, as disclosed in Japanese Patent Application Laid-Open Publication No. 2009-154693 (Patent Document 1). The cushion frame for a vehicle seat disclosed in Patent Document 1 comprises a lower arm having a bead portion (a recess). A bend point in a dogleg shape when viewed from the side is formed on the rear side on the periphery of the lower arm. The bead portion is formed on the inside surface or outside surface of the lower arm and has an upper edge extending from the bend point. By the constitution described above, when a large backward force in applied to the seat back frame while a bending moment is applied to the reclining mechanism, the rear side of the lower arm forms a compressed shape in the vertical direction, thereby absorbing the collision energy.